1. Field of the Invention
The present invention relates to a ceramics binder mixture suitable for binding low thermal expansion cordierite ceramics or the like, and a binding method wherein this ceramics binder mixture is used.
2. Discussion of Background
As binder mixtures useful for binding ceramics of the same or different kinds to be used at high temperatures, various binder mixtures are commercially available depending upon the particular purposes and applications. Such binder mixtures include those which utilize hardening of cement, those which utilize solidification by a dehydrating condensation reaction of an alkali metal silicate solution and those which utilize solidification by a dehydrating condensation reaction of a phosphate solution.
Usually, such binder mixtures comprise a binder component to provide binding strength, a filler to reduce the difference in the average thermal expansion coefficient between the binding layer and the material to be bound and to reduce the shrinkage of the binding layer upon drying, a plasticizer to improve the workability of the composition, and additives to adjust the dispersibility or the hardening rate of the binder mixture. However, there has been no ceramics binder mixture which is reliable and which has practical binding strength up to a high temperature range exceeding 600.degree. C.
The present inventors have previously proposed in U.S. Pat. No. 5,073,178 a filter tube made of a low thermal expansion cordierite ceramics having a high porosity and practical strength even at a high temperature of exceeding 600.degree. C., as a filter for removing dust from a high temperature dust-containing gas. U.S. Pat. No. 4,584,003 proposes a filter apparatus provided with a filter tube of such a type. Further, Japanese Examined Patent Publication No. 20269/1982 proposes a process for producing low thermal expansion cordierite aggregates used as the main material of the cordierite ceramics filter as disclosed in U.S. Pat. No. 5,073,178. In the process for producing cordierite aggregates, a mixture material having an approximate cordierite composition (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) is melted; the melt is rapidly cooled to obtain glass particles, and the glass particles are heated again for crystallization to cordierite.
The cordierite aggregates prepared by this process and a cordierite ceramics prepared by sintering the cordierite aggregates, have a characteristic such that the mean thermal expansion coefficients between room temperature and 1,000.degree. C. (hereinafter referred to simply as a mean thermal expansion coefficient) can be made to be not more than 20.times.10.sup.-7 /.degree.C. with good reproducibility, even if the cordierite crystals have no orientation i.e. even without molding by extrusion to form a thin walled honeycomb structure, while the mean thermal expansion coefficient of usual cordierite ceramics is from 22.times.10.sup.-7 /.degree.C. to 26.times.10.sup.-7 /.degree.C.
The cordierite filter tube which can be produced by the process disclosed in U.S. Pat. No. 5,073,178, is limited in its length for a practical reason. For example, it is difficult to produce a filter tube having a length of 1 m, since such a tube is likely to undergo deformation by its own weight during the firing step. Yet, it is advantageous to use long filter tubes, when the economy of the filter apparatus for removing dust from a high temperature dust-containing gas is taken into consideration. Therefore, a long filter tube having plural (e.g. three) elemental filter tubes of a short length (e.g. 710 mm) bound to one another, is practically used. In such a case, to secure the reliability of the filter tube, it is essential that the bound areas of the filter tube have sufficient binding strength and reliability. Namely, it is essential that the bound areas will not be damaged by the mechanical shock exerted at the time of mounting the filter tube to the filter apparatus, by the thermal shock or thermal stress due to the temperature rise and drop of the high temperature dust-containing gas to be treated, and by a stress caused by e.g. vibration during its use.
For binding elemental filter tubes, using a cylindrical joint made of cordierite ceramics, various conventional ceramics binder mixtures including commercial products have been tried, but with conventional binder mixtures, it has been impossible to obtain sufficient and reliable binding strength. Namely, with conventional binder mixtures, the binding layers of the mixtures undergo cracking due to shrinkage resulting from drying or solidification, and constant binding strength can hardly be obtained due to fluctuation in strength. Further, when at least one of the objects to be bound is porous, the liquid component in the binder mixture tends to penetrate into the object to be bound, and cracks are likely to form at the boundary between the binding layer and the object to be-bound, whereby it has been difficult to obtain sufficient and reliable binding strength.
Further, Japanese Unexamined Patent Publication No. 192176/1991 discloses a binder mixture useful for binding a cordierite ceramics comprising cordierite aggregates and fumed silica. However, the mixture contains sodium silicate and thus has a problem that the Na.sub.2 O component reacts with the cordierite aggregates at a high temperature, whereby the cordierite aggregates undergoes a property change, and the mean thermal expansion coefficient of the cordierite aggregates or the ceramics containing the aggregates, will increase, and the binding strength at a high temperature will be low.